The use of passive porosity is well known in the art as a method for improving the aerodynamics and/or performance of an aircraft or other air vehicle. Generally, passive porosity comprises the application of a set of fixed openings or pores to one or more surfaces of the aircraft and may typically also include a recirculation cavity or plenum chamber located beneath the region of openings or pores. The pores and the plenum chamber allows for a conditioning of the air stream boundary layer as it passes over the porous aerodynamic surface. The plenum chamber allows the air stream to transition from a high pressure region of the aerodynamic surface to a lower pressure region.
Although its use is generally limited, one of the more common applications of passive porosity on aircraft is at the air intake or inlet of a jet engine. For example, some fighter aircraft employ passive porosity as a set of fixed holes or pores formed at the engine inlet to condition the air prior to entry into the engine. For non-porous surfaces, the boundary layer at the engine inlet may become disturbed at certain flight conditions which can disrupt the aerodynamic flow into the engine inlet. However, by providing sections of pores at strategic locations on the engine inlet, the disturbed airflow is conditioned prior to entry into the engine resulting in an improvement in engine performance.
The effects of passive porosity are documented in the reference “Computational Analysis of Drag Reduction and Buffet Alleviation in Viscous Transonic Flows Over Porous Airfoils,” by Mark A. Gillan, (AIAA-93-3419) which indicates that for a given design condition, the application of passive porosity can weaken shock and improve aerodynamic efficiency. For example, when applied to a wing surface, passive porosity alters the normal shock to a lambda shockwave which spreads or distributes the shockwave over the porous region. The net effect of the lambda shockwave is an improvement in aerodynamic efficiency with a reduction in overall drag and a reduction in boundary layer thickness aft of the shockwave.
Unfortunately, the Gillan reference also concludes that while passive porosity produces a reduction in total drag above certain Mach numbers, for Mach numbers below a certain value, the porous surface actually resulted in an increase in drag compared to a solid or non-porous surface. For example, Gillan indicates that while porous surfaces reduced drag by 26 percent for Mach numbers greater than 0.79, the same porous surface produced greater drag for Mach numbers less than 0.77 as compared to a non-porous or solid surface.
Another prior art reference indicates that passive porosity can be applied to certain areas of the aircraft in order to reduce the acoustic signature or noise generated during certain flight conditions. For example, it is well known that trailing edge flap systems, when deployed, are contributors to landing noise. Such noise is typically generated as a result of vortices interacting with the flap. A reference entitled “Trailing Edge Flap Noise Reduction by Porous Acoustic Treatment,” (AIAA-97-1646) by James D. Revell et al. indicates that wind tunnel testing revealed significant reductions in noise due to the application of passive porosity to portions of the flap. However, Revell further postulates that despite the noise reduction, there may be cruise drag penalties associated with the porous surfaces of the flap at cruise flight conditions.
As can be seen, the ability to apply passive porosity to aircraft has been limited due to the penalties imposed at off-design conditions. As such, there exists a need in the art for a system and method for varying the porosity of an aerodynamic surface such that the porosity provides performance, economy and environmental advantages through a wide variety of flight conditions. Furthermore, there exists a need in the art for a system and method for varying the porosity of an aerodynamic member that is of simple construction and of low cost.